Cellular Biophysics of Active Salt Transport: Details of the sodium-transport system of Toad bladder will be explored by determination of transference numbers of ions in the system, as preliminary to tests of a new model of the Na/K ATPase pump mechanism. Studies of behavior of frogs, with respect to emergence from and submergence in water, in relation to condition of hydration and of salt balance will be carried out. Radio-immune assay will be used to estimate concentrations of arginine vasotocin in frog plasma in relation to state of hydration and of salt balance. Kidney function in frogs in relation to salt and water balance: Estimates of glomerular filtration rate and renal tubular flow, using accepted techniques, will be accompanied by estimation of rates of reabsorption rates for Na and water in renal tubules and bladder of frogs under known conditions of salt and water balance. Systems analyses of metabolism of water, salts, and carbohydrates: Analyses now in progress of salt and water balance in frogs, and of carbohydrate metabolism in crustaceans, will be completed, and generalized to include physiological information from human beings, with the intent of preparation of computer programs useful in clinical evaluations of metabolic state.